Mach-Zehnder filters are optical waveguide devices used in a variety of optical communications applications. A typical Mach-Zehnder filter comprises a pair of optical waveguides (interferometer arms) extending between two coupling regions. In the coupling regions, the cores of the respective waveguides are disposed sufficiently close together to permit evanescent-field coupling. In the region between the couplers, the waveguides are disposed sufficiently far apart that no evanescent coupling occurs. Each of the interferometer arms includes a refractive index Bragg grating, and the two interferometer arms are conventionally of equal length.
An improved Mach-Zehnder filter is disclosed in the U.S. Pat. No. 5,636,309 issued to applicant C. H. Henry et al. on Jun. 3, 1997. In this improved filter the spaced apart arms are sufficiently close together that a Bragg grating can be simultaneously written in both arms, providing nearly identical gratings in both arms. The patent further describes articles employing the new filters such as add-drop filters and wavelength division multiplexers (WDMs).
One shortcoming of conventional Mach-Zehnder filters is that ideal operation can be achieved only approximately. Significant causes for non-ideal operation are imperfect 3 dB couplers, non-identical gratings in the two interferometer arms, and inadvertently unequal path lengths. The device of the aforementioned U.S. Pat. No. 5,636,309 patent substantially eliminates non-identical gratings, but ideal operation strongly depends on precisely balanced 3 dB couplers. Accordingly there is a need for a new Mach-Zehnder filter of reduced coupler dependence.